The present invention relates generally to solar arrays employed on satellites, and more particularly, to control systems and methods that controls the solar arrays so that they point at the sun.
State of the art relating to pointing of solar arrays falls into two categories. In a first category, the satellite attitude is generally Earth fixed and the orbit is near circular. In this case, the solar array pointing can be synchronized to a simple clock. In a second category, the satellite attitude is not Earth fixed, or the orbit is elliptical. In this case, the array pointing is performed using knowledge of the satellite orbit and attitude with respect to the sun.
Concentrator-type solar arrays that require tight pointing with respect to the sun have special requirements. A satellite that requires self-detecting fault detection, isolation and recovery (FDIR) also has special requirements. The present invention addresses the problem of synchronization to a simple clock, as posed by the application of a concentrator-type solar array, as well as the problem of recovery after a fault detection for the application of a solar array that is pointed using knowledge of the satellite orbit and attitude.
It is believed that conventional control schemes have described in the literature that use power (not current) as a control input. The inventors are also unaware of conventional control schemes that use both current and position estimates and use weighting/logic to determine when to step the array based on factors such as fault detection and eclipse periods, and the like.
Accordingly, it would be advantageous to have improved control systems and methods that control solar arrays disposed on a satellite so that they point at the sun.
The present invention provides for a control system and method that keeps solar arrays pointing toward the sun. The present invention is especially useful with concentrator-type solar arrays and/or periods of loss of fine pointing control of the satellite bus.
An exemplary control system comprises an orbit propagator that computes and outputs real-time satellite orbital location data and a sun vector referenced to an inertial reference frame. A coordinate transformation processor processes the sun vector in the inertial reference frame and the satellite attitude to generate a sun vector referenced to a satellite body reference frame. A solar array position processor processes the requested solar array step count taken from a solar array pointing control system and the sensed solar array position derived from a solar array position sensor to generate an estimate of the solar array position.
A summing device processes the solar array position estimate from the solar array position processor, the body frame sun vector, and a bias signal (which generally is zero) from ground command, to produce a solar array position error signal. A filter filters the solar array position error signal output by the summing device such that the output signal has most noise sources removed.
A current regulator (K/xcfx84S) processor processes the measured solar array current signal to estimate a solar array position error signal. The current regulator has a sufficiently long time constant (xcfx84) such that the output signal has most noise sources removed. An eclipse processor uses the satellite orbital location output by the orbit propagator, the estimated solar array position error output signal derived from the current regulator processor, and the low pass filtered solar array position error output signal to generate a step command (xcex4STEP) The eclipse processor uses logic to determine which solar array position error is more heavily weighted. This weighting can change per ground command, or autonomously during periods when the satellite is in an eclipse period, which would render the solar array position error from the current regulator meaningless. The weighting can also change during periods that a fault detection declares that orbit or attitude data are unreliable, which would render the solar array position error from the position estimate meaningless. The solar array pointing control system processes the step command to generate control signals that point the solar array at the sun.
An exemplary control method comprises the following steps. Orbit location is computed. A sun vector referenced to an inertial reference frame is determined. Satellite attitude is sensed. The sun vector referenced to the inertial reference frame and the satellite attitude are processed to produce a sun vector referenced to a satellite body reference frame. The requested solar array step count taken from a solar array pointing control system and the sensed solar array position from a solar array position sensor are processed to compute a solar array position estimate.
The solar array position estimate, the body frame sun vector, and a bias signal from ground command, are summed to produce a solar array position error signal based on an estimate of the position. The solar array position error signal filtered. The solar array current is sensed and processed by the current regulator to produce another solar array position error signal, but based on the current output and not a position estimate.
The satellite orbital location, the solar array position error signal based on current output, and the solar array position error signal based on the position estimate are processed to generate a step command. The step command is processed by the solar array pointing control system to point the solar array at the sun.
When used with a concentrator-type solar array, the present invention does not require adjustment to the clock or to the position of the array, as is required if simple clock synchronization is utilized. The present invention also does not require knowledge of the satellite attitude or orbit during recovery from fault detection when used with any type of solar array.